1. Field of the Invention
The present invention relates to liquid-crystal display apparatuses, and more particularly, to liquid-crystal display apparatuses employing an active-matrix addressing method.
2. Description of the Related Art
In color liquid-crystal display (hereinafter called LCD in some cases) apparatuses which employ an active-matrix addressing method, a plurality of color pixels, each showing one color by combining a number of basic colors, are arranged in a matrix. The color pixels are matrix-addressed with scanning lines (gate buses) and signal lines (source buses).
A technology has been proposed for such LCD apparatuses, in which a combination of basic colors, such as the three primary colors, red (R), green (G), and blue (B), is repeatedly arranged in a direction along each signal line. The number of the signal lines is set to the number of the basic colors multiplied by the number of pixels in the direction along a signal line. The number of the basic colors is typically set to three. A scanning line method employing such a structure is generally referred to as a “tripled scanning line method.” In conjunction with tripled scanning line method, another method, referred to as 3:1 interlaced driving, is sometimes employed. In 3:1 interlaced driving, only one in every third line of a display is scanned at a time.
In a tripled scanning line method, the number of gate drivers is three times as large as that used in a conventional scanning line method. However, source drivers consume more power and are more expensive than the gate drivers. Therefore, the power consumption of using a tripled scanning line method is cut to one-third the power consumption of using a single scanning line method and the cost of the LCD apparatus is commensurately reduced.
The use of the 3:1 interlaced driving also reduces the power consumption of a LCD apparatus. In 3:1 interlaced driving, the frame frequency (frequency at which one entire screen is rewritten) is cut to one-third compared to using conventional interlaced driving.
There are some drawbacks, however, of using 3:1 interlaced driving methods. Due to the reduced frame frequency, for example, movement becomes less smooth in some cases when moving images are displayed. This display unevenness is referred to as “line crawling.”
In general, liquid-crystal polarity inversion driving methods include dot inversion methods, which emphasize image quality, and common inversion methods, which emphasize power reduction. Dot inversion methods reduce line crawling by reducing the distance D between lines when dots having the same primary color (such as G among R, G, and B) being driven by the same-polarity driving voltage are connected to each other. For example, in a LCD apparatus that has a viewing distance of 30 cm, the line distance D is preferably 260 μm or shorter. In this example, line crawling can be reduced when dot inversion driving is used,.
A system which employs the tripled scanning line method and interlaced driving, described above, is suited to portable terminals and other devices where power reduction and low cost are of concern but motion-image display performance is less of a concern. In such a system, it is preferred that the common inversion method be used since it is more effective for reducing power than the dot inversion method.
The use of common inversion driving, however, can be problematic in some cases when it is employed in conjunction with 3:1 interlaced driving and tripled-scanning-line-methods. For example, problems can occur if the distance D between a plurality of lines obtained when dots having the same basic color and being driven by the same-polarity driving voltage are connected to each other in their vicinities is 6P (where P indicates the pitch of color pixels each formed of three dots).
FIG. 29 is a view for explaining the above, and shows dots arranged in a matrix manner in 30 rows. Letters A, B, and C placed at the left-hand side of the figure indicate write timing in 3:1 interlaced driving. For example, first, data is sequentially written into rows having A from the top to the bottom, then, data is sequentially written into rows having B from the top to the bottom, and finally, data is sequentially written into rows having C from the top to the bottom. Since R, G, and B are arranged periodically in the vertical direction, the rows having A, B, and C are not periodically arranged so as to prevent only the same basic color from being always written at timing A. In common inversion driving, all dots arranged in each row horizontally have the same polarity. When the basic colors, R, G, and B, are arranged in that order repeatedly from the top row to the bottom row in FIG. 29, the fifth row has negative-polarity G dots written at timing A, and the next negative-polarity G dots appear at the 23rd row. Therefore, the line distance D corresponds to 18 dots, that is, six pixels.
The color pixel pitch P is, for example, 127 μm at a pixel density of 200 pixel per inch (ppi), which is generally said to be a high definition. The corresponding line distance D is D=6P=762 μm. The line distance D is long enough to visually recognize line crawling. In a 3.5-inch QVGA (320 by 240 pixels) display unit, which is popular for current portable terminals, P is 223.5 μm and D=6P=1,341 μm, which will undoubtedly result in recognized line crawling. Conversely, to set the line distance D to 260 μm or shorter, the color pixel pitch needs to be 43 μm or less, which is currently difficult to produce in the making of high-pixel-density display units.
In other words, when common inversion driving is employed together with the conventional tripled-scanning line method, it is difficult to apply a sufficient countermeasure to line crawling.
When dot inversion driving is employed, which reduces line crawling, adjacent dots arranged in each row horizontally have opposite polarities as shown in FIG. 28, unlike common inversion driving. In this case, the line distance D is 1.9P. At a pixel density of 200 ppi (P=127 μm), D is 241.3 μm, which is shorter than 260 μm. Thus, an effective countermeasure against line crawling can be implemented. In dot inversion driving, however, the signal amplitude is about twice that used in common inversion driving. Therefore, power consumption increases since the power consumption of using only source drivers is about four times as large as in common inversion driving.